In general, dot matrix printers can be separated into two categories--dot matrix line printers and dot matrix serial printers. Both categories of printers create images (characters or designs) by selectively printing a series of dots in an X-Y matrix. A dot matrix serial printer includes a head that is moved horizontally back and forth across a sheet of paper, either continuously or in steps. The head includes a vertical column of dot printing elements. As each column position of a character position is reached during printing, the required number of dot printing elements are actuated to form dots. A series of thusly created vertical dot columns forms the desired character. Contrariwise, dot matrix line printers include dot printing mechanisms for creating horizontal lines of dots substantially simultaneously as paper is stepped through the printer. A series of horizontal lines of dots creates an image, i.e., a row of characters or a design. The present invention is related to dot matrix line printers, as opposed to dot matrix serial printers.
In the past, various types of dot printing mechanisms for use in dot matrix line printers have been proposed and implemented. U.S. Pat. No. 4,351,235 assigned to the assignee of the present application and entitled "Dot Printing Mechanism for Dot Matrix Line Printers" describes a dot printing mechanism for a dot matrix line printer comprising a plurality of hammer modules mounted on a carriage that is shuttled back and forth along a print line. Each module includes a plurality of cantilevered print hammer arms formed of a resilient ferromagnetic material. Each print hammer arm includes two pieces--a thin resilient piece and a stiffener mounted on the end of the resilient piece. Mounted on the end of the stiffener is an anvil that prints a dot when the associated hammer arm is actuated.
Associated with each hammer arm of the dot printing mechanism described in U.S. Pat. No. 4,351,235 is a permanent magnet, a post and plates that create a ferromagnetic path between the permanent magnet and the post. The post supports a coil and is positioned near the stiffener end of the print hammer, on the side opposite the anvil. In the absence of current through the coil, the print hammer is attracted to the post by the magnetic field produced by the permanent magnet and, thus, cocked. The cocked hammers are released to create dots by energizing the coils such that the coils produce a magnetic field that counteracts the magnetic post attraction field created by the permanent magnet.
While dot printing mechanisms for dot matrix line printers of the type described in U.S. Pat. No. 4,351,235 have a number of advantages over prior dot printing mechanisms for such printers, and, thus, form a significant step forward in this art, it has been found that such dot printing mechanisms can be improved. In this regard, as noted above, the dot printing mechanism described in U.S. Pat. No. 4,351,235 includes a two-piece hammer arm. A two-piece hammer is undesirable because it is expensive to produce. Two pieces must be formed and welded together. Another disadvantage of such dot printing mechanisms results from the hammer impinging only on the tip of the coil post. Since the pole tip is small in size post wear is high, whereby the life of such dot printing mechanisms is shorter than desired. The present invention is directed to overcoming these disadvantages.